1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor light emitting device. More particularly, the present invention relates to a method of manufacturing a semiconductor light emitting device having high reliability even at a high temperature such as in case of a semiconductor laser.
2. Description of the Prior Art
The inventors of the present application found, as a result of recent studies, that one of the important conditions for obtaining semiconductor light emitting device having high reliability at high temperature is that such an interface as described below must not be formed. More specifically, if a grown layer is provided on a surface exposed to a high temperature atmosphere to form a p-n junction in the crystal interface, a semiconductor light emitting device having such an interface (referred to hereinafter as the exposed p-n junction interface) is in a state where the forward direction voltage in the exposed p-n junction interface is gradually lowered (deteriorated) when forward current is supplied in this junction. It was ascertained that such deterioration becomes more excessive according to the elevation of the conduction temperature or the increase of the current density in conduction. Based on this fact, the inventors of the present application issued a paper in the Applied Physics Letters 43 (2), 15, pages 187 to 189 (issued in July, 1983) concening a deteriorated position of an n substrate BC (buried crecent) laser and an improved structure thereof. This paper proposed a structure of a BC type semiconductor laser in which the exposed surface and the p-n junction interface are separated.
Now, description will be made of a structure of a semiconductor light emitting device called a BC type semiconductor laser and the manufacturing process thereof, with reference to FIGS. 1, 2A to 2C, 3A and 3B.
First, referring to FIG. 1, a BC type semiconductor laser comprises a p-side electrode 1, an n-side electrode 2, an n-InP substrate 3, a P-InP layer 4, an n-InP clad layers 5 and 5a, an n or p-InGaAsP active layers 6 and 6a and a p-InP clad layer 7. The BC type semiconductor is manufactured by the process shown in FIGS. 2(A) to 2(C). More specifically, as shown in FIG. 2(A), a p-InP layer 4 is formed on a semiconductor substrate 3 by the first growth or diffusion. Then, as shown in FIG. 2(B), a groove 8 shaped like a stripe is formed in the substrate 3 and the p-InP layer 4. After that, as shown in FIG. 2(C), an n-InP clad layer 5, an n or p-InGaAsp layer 6 and a p-InP clad layer 7 are formed successively by second growth in the regions covering the inner and outer portions of the groove 8.
In this case, referring to FIGS. 3(A) and 3(B), assuming that the carrier concentration of the p-type impurity in the p-InP clad layer 7 is Np and that the carrier concentration of the n-type impurity in the n-InP clad layer 5 is Nn, the values of Np and Nn are set to satisfy the condition of Np&gt;Nn. As the p-type impurity, Zn, for example, is selected for the purpose of facilitating the diffusion. Under these conditions, at the time of growth of the p-InP clad layer 7 in the inner and outer portions of the groove 8 or by the heat treatment after the growth, diffusion of the impurity is caused so that the positions A, B, C and D of the p-n junction interfaces initially exposed to the atmosphere shown in FIG. 3(A) can be moved to the positions A', B', C' and D' in the n-InP clad layer 5 as shown in FIG. 3(B).
By moving the p-n junctions from the exposed interfaces by the diffusion as described above, such exposed p-n junction interfaces recognized as a factor of deterioration of the electric characteristics at the time of operation at high temperature are made to disappear. As a result, by such movement of the interfaces, continuous oscillation can be applied at a high temperature, say 80.degree. C., though conventionally it was difficult to apply continuous oscillation at a temperature higher than 60.degree. C. However, through a closer examination of the oscillation characteristics at high temperature, it was made clear that deterioration of the characteristics cannot be perfectly prevented.
Specifically stated, when a constant light output operation experiment was performed for several thousands of hours with the conditions of 80.degree. C. and light output of one surface of 5 mW, increase of the oscillation threshold current, namely, deterioration was found. Upon investigation of the cause of such deterioration, it was made clear that dotted (linear in the depth direction) deteriorated portions at the intersection between the exposed interfaces and the moved p-n junction interfaces exist in the portions shown by the points X in FIG. 3(B) and that leakage of current occurs in those portions.